I have 6 yo and an 8 yo, and if either of them did that to me, First I'd say a loud, "HEY! You really shouldn't leave that there." (not mean, just loud) If that didn't work either I would know something was up so I'd walk over to wherever they were, make sure I got down to their level and say something like, 'Hey there - I need your attention for a minute. You need to go pick up that stuff, you know it's not OK to just drop things on the floor like that. After that, let's talk about what's up."
I don't know - I mean, my kids drop stuff ALL THE TIME. They're lazy like me and want to get away with whatever they can. Usually, I just stand over whatever it is, call their name and look down at the floor, and they say, "Oh uh, oops, let me get that." or something like that. LOL. I've not really had to deal with a lot of walking away/ignoring much like that.
I *might* suggest being more matter of fact and less like you're asking a favor, if you know what I mean. Sometimes, I use bemused disbelief as a kind of humor/reality check for them now that they're older. Like, "REALLY? You're going to drop paper on the floor and walk away from it?" - I don't say that, and I'm not *rude*, but more like a good natured, "Come on, you're old enough to know better than that, and are capable of taking care of your own messes."
All of that goes along with convos about being responsible for your stuff, about teamwork to keep the house safe and comfy, etc. So it's not like it's coming out of nowhere. They've both been doing chores for a couple years now so it's not like it's a new concept to them.